High-frequency short-circuiting arrangement



April 29, 1952 E, `-*EIKERJR 2,594,895

HIGH-FREQUENCY SI'ORT'CIRCUI'IING ARRANGEMENT Filed DBG. 21, 1946 Inventor: Geerge E. FeikenJrr l-Iis Attorn.

Patented Apr. 29, 1 952 HIGH-FREQUENCY SHORT-CIRCUITING ARRANGEMENT` George E. Feiker, Jr., Albany, N. Y., assignor to General Electric Company, a corporation of New York Application December 21, 1946, Serial No. 717,711

My invention relates to high frequency circuits and, in particular, to a tunable short-circuitfor resonant cavities of the transmission line type.

As is well known in high frequency circuits, it is usual to employ concentric transmission lines for conducting energy and, in order to reduce eX- cessive losses and erratic operation due to metallic contacts when it is desired to obtain a shortcircuit at a point, to employ a section of coaxial line which is resonant at the frequency of the energy being transmitted and which functions to change a high impedance at one end of the section to a low impedance at the other end. Such an arrangement is well adapted for usev at frequencies of the order of 2000 megacycles or greater. However, at lower frequencies, the physical length of the transmission line section which is equal to an electrical quarter wave or a multiple thereof becomes excessive and, as a result, it has been customary to employ metallic finger contacts at lower frequencies with consequent sparking and contact diiiculties.

It is an object of the present invention to provide a new and improved short-circuiting arrangement for concentric transmission lines translating waves whose frequency is considerably less than 2000 megacycles.

It is another object of my invention to provide a new and improved high frequency choke.

The present invention is principally concerned with the provision of an improved form of high frequency circuit which is characterized by simplicity and compactness of structure and which is well adapted for use at frequencies of the order of 1000 megacycles or less, but which is adapted for use at even shorter wavelengths.

Briefly, the invention makes use of la re-entrant radial transmission line which is disposed between the conductors of a concentric transmission line and which has a longitudinal length considerably smaller than a quarter wavelength at the frequency of waves transmitted over the line. The short-circuiting device may employ two or more serially connected sections of radial transmission line, the elements constituting the sections having substantially no contact with one or both of the conductors of the concentric transmission line.

The novel features which I believe to be characteristic of my invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof,

may vbest be understood by reference to the fol- 1 Claim. (Cl. 178-4'4) elevation view, partly in section, of a high frequency wavemeter embodying -one of my improved short-circuiting arrangements; Fig. 2 is a longitudinal sectional view of a portion of a concentric transmission line including a modication of my short-circuiting arrangement; and Fig. 3 is a longitudinal sectional View of another modication of my invention particularly adapted for use in wide band transmission of high frequency energy.

Referring particularly to Fig. 1, there is shown a high frequency wavemeter which comprises a cavity resonator I of theffolded coaxial transmission line type having an input coupling loop 2 and an output coupling loop 3. Energy picked up by the output loop 3 is rectified by rectifier 4 and supplied to meter 5, the maximum reading on the meter 5 indicating a resonance condition of the resonatorl. The active parts of the wavemeter may be enclosed in a container 6 which serves as a housing for the resonator and crystal and asupport for the meter.

The cavity resonator I is illustrated as comprising a tubular outer conductor 'I and a tubular inner conductor 8 which is centrally positioned within the conductor 1 and supported from an end wall 9. The transmission line resonator is re-entrant in character having a metallic rod I0 which may be extended an adjustable distance into the tubular inner conductor 8. The rod I0 may comprise the spindle of a micrometer having an external adjusting knob II and the usualcooperatingl scale elements I2, I3 for indicating the extension of the spindle or rod` I0 into the tubular conductor 8. The coupling loop 2 comprises the inner conductor of a concentric transmission line coupling having a tubular outer conductor I4 extending out of the housing 6. ured may be supplied to the wavemeter by connecting a concentric transmission line (not shown) to the elements 2, I4 of the high frequency coupling. When the wavemeter illustrated is to operate to measure frequencies over a reasonably broad range, such as, for example, the range of 780 to 920 megacycles, I have found that the length of the resonator formed by the elements 1, 8, for satisfactory operation, should be not more than electrical degrees; that is, the right-hand end wall I5 of the resonator is spaced' from the end wall 9 by a distance equal to'not more'than 60 electrical degrees, the remaining electrical length of the resonator being constituted by the re-entrant section of theline comprising rod I0 and conductor 0.

High frequency energy to be meas- With such a construction, the current at the point of contact between the rod I0 and the end wall I5 would be equal approximately to 50% of the maximum current within the resonator. As a result, it is undesirable to rely upon metallic contact between the rod I0 and any structure affixed to the end Wall I5 to conduct a current of such magnitude. Accordingly, I provide a non-contacting short-circuiting arrangement between the end Wall I5 and the rod I0 which comprises a re-entrant radial transmission line section. A longitudinally extending sleeve I5 is affixed to the end wall I5 and, at its outer-end, is provided with a radially extending disk' I-`I spaced from the outer surface of theend wall I5. A cup-shaped member I8, adapted to iit within a shoulder I9 on the right-hand end of outer conductor 1, provides a disk-like surface which is in opposed relation with the right-hand sur.- face of the disk I'I. The member I 8 may likewise function to support a bearing member 2% for the spindle or rod I0.

The elements IE-IG provide. a rfc-entrant, radial transmission line which consists ofA three sections. The first section is constituted by the opposed surfaces of the disk I'I and the member I8. rIne second section is constituted by the. opposed surfaces of the member I8 andv the end wall I5 at the outer edge of the disk I'I. The third section comprises the opposed surfaces of the end wall I5 and the disk II. The second and third of these sections provide an inductive reactance which is used to terminate theV rst section constituted by the opposed surfaces of the members I1, i8. While these-sections actu- 1` ally are radial transmission lines having distributed impedance, in an equivalent circuit they may be represented as a lumped inductance or capacitance, as theV case may be; Thus, the. iirst section functions essentially as a lumped capacitance tuning the input impedance across the gap between the sleeve I6 and the bearing member 2B to a maximum value. In accordance with my invention, the lumped capacitance provided by the radial transmission line I'I, I8. is made. variable for adjustment in testing the arrangement for satisfactory operation. To this end, the longitudinally extending part of the member I8 fits into the shoulder I9 an adjustable distance so that the spacing between the opposed surfaces of the disk Il and the radial portion of the member I8 may be adjusted to the value which gives desired operating characteristics. Thereafter, if necessary, a spacer may be placed between members I5 and 2S to assist in supporting the end wall I5.

In the operation of the Wavemeter of Fig. l, input energy, the frequency of which is to be determined, is supplied to the cavity resonator by means of the coupling loop 2. The resonant frequency of the resonator I is adjusted by means of the knob II which controls an extension of the rod or spindle I0 into the tubular conductor 8. In effect, the portion of the rod I0 within the conductor 8 constitutes a variable capacity which resonates with the series inductance provided by the xed line constituted by the conductor 8 and the cuter conductor l. Coupling loop 3 supplies energy from the resonator to the rectifier 4, which may be, for example, of the crystal type. Rectiiied current may be supplied to the microammeter 5 over a conductor 2l, the return path for the current being through a ground connection.. Resonance of the resonator l at the frequency of 75 the opposed surfaces of the bearingZ are made,

the energy supplied thereto is indicated by the peak reading of the microammeter.

The re-entrant radial transmission line constituted by the elements I5-I8 provides a large impedance at the right-hand end of the coaxial transmission line constituted by the opposed surfaces oi the rod I0 and the sleeves I6. Thus, on

anequivalent circuit basis, the three radial transmission line sections constituted by members I 5-I8 are equivalent to a parallel L-C tuned circuit. The coaxial line I0, I6 through which energy passes from the resonator I to the reentrant radial transmission line sections is usually considerably less than a quarter wavelength at the, resonant frequency of the resonator because of limitations of physical size of the wavemeter when constructed for measuring waves of the order of 1000 megacycles or less. Since it is desirable to present a low impedance at the lefthand end of coaxial line I0, I6, the line I0, I5 preferablyisof the lowimpedance type. Because the length of `this, line, moreover, is always less than a quarter wavelength, the input impedance at the left-hand junction of the members I0, IG is always capacitive. It is apparent, therefore, that the large currents which are required to be conducted between `the end Wall I5 and the reentrant rod I0 are transmitted between these elements as capacitive currents Without excessive losses or erratic operation and without requiring any physical contact between the sleeve I6 and the rod I0. To this end, the sleeve I6 preferably loosely surrounds the rod I0.

In Fig. 2, I have shown a modification of the arrangement of Fig. l for short-circuiting a coaxial transmission line. In this arrangement, the transmission line to be short-circuited comprises the tubular outer conductor 22 and the centrally disposed inner conductor 23. Assuming that the right-hand end of the line is connected to a source of energy and that a high impedance is to bev provided betweeny a bearing 24 and both of the conductors 22, 23, 1 provide a longitudinally extending sleeve 25 secured to the bearing 2d. At the right-hand end of the sleeve 25, there is ccnnected a tranverse wall member 26. At the inner and outer edges of the endsof the wall 26 are provided longitudinally extending conductive members 2l, 28. The left-hand ends of the members 21, 2 8 are provided, respectively, with flanges t 29;, 3 0 extending toward the transverse sleeve 25..

In this construction, the members Zit-26, 29 provide three re-entrant sections of radial transmission line which function as a choke for currents along the conductor 23. The members 2li-2S, 30 provide three re-entrant sections of radial transmission line which constitute a choke for currents along the outer conductor 22. The surfaces of members 2, 28 adjacent the transverse member 25 function as short-circuiting elements for these serially connected radial transmision line sections. In this arrangement, a high impedance is established at the junction of the bearing 24 with both conductors 22, 23. The coaxial lines established, respectively, between conductor 22 and the member 28 and conductor 23 and member 2, as pointed out previously, are low impedance lines whose length is less than an electrical qua-rter wavelength so that the input impedance of each or" these lines is capacitive.

In the construction of Fig. 2, since the radial transmission line sections are to resonate at the same frequency, the radius of .the sleeve .25 and the spacing of the walls 29, 30 with respect to such that these inner and outer chokes, constituted respectively by members 24-21, 29 and members 24-26, 28, 30, resonate at the same frequency.

- In Fig. 3, I have shown a modification of my short-circuiting arrangement which is suitable for circuits operable over a wide band of wavelengths. In this arrangement, two or more radial transmission line chokes are arranged in cascade and tuned to diiTerent frequencies in the band of frequencies to be covered. The two chokes are connected in cascade at the end of a transmission line constituted by the outer conductor 3| and the centrally positioned inner conductor 32 and closed at its left end by an end wall 33. Assuming that it is desired to obtain a high impedance between the end wall 33 and conductor 32 overa band of frequencies, I provide a rst choke comprising serially connected sections of a radial transmission line constituted by a pair of disks 34, 35 connected together by a longitudinally eX- tending sleeve 36 closely spaced to the inner conductor 32. The disk 34 is conductively connected to the outer conductor 3| in any suitable manner and disk 35 is spaced from the inner surface of end wall 33. The dfnensions of the radial unes between members 34, 35 and 33, 34 and the spacing between the opposed surfaces of the members 33, 35 are adjusted to resonate at a frequency near one end of the band of frequencies transmitted over the line 3|, 32. connected radial transmission line choke is constituted by disk members 34, 31, 38 and longitudinally extending sleeve 39. The member 31 is conductively connected with the`outer conductor 3| and the member 38 is spaced from the disk 34. The spacing of the disks 3l, 38 and the spacing between the disks 34, 38 are adjusted so that this choke resonates at a frequency in the opposite end of the band of frequencies transmitted over line 3|, 32. A sleeve 40, which may be formed integral with the sleeve 39, extends along the conductor 32 to provide a low impedance input line to the cascaded resonant chokes. In such an arrangement, by properly controlling the spacing between the sets of disks and the capacity established by the outer disk with the adjacent eiective end wall, an extremely broad band characteristic may be obtained. Such an arrangement may be used, in fact, for a transmission A second seriallyline system where the highest frequency transmitted is as much as three times the lowest frequency transmitted. While in Fig. 3, I have shown an arrangement employing two cascaded chokes, it is apparent that more may be employed where required.

While the invention has been described by reference to particular embodiments, it will be understood that numerous modications may be made by those skilled in the art without departing from the invention. I, therefore, aim in the appended claim to cover all such equivalent variations as fall within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

A high frequency short-circuiting device for a transmission line of the type having concentric inner and outer conductors comprising three conductive walls arranged in parallel planes transverse to said conductors, the outer of said walls being connected by one of said conductors, on' of said outer walls and the inner of said walls being connected by a conductive cylinder closely spaced to the other of said conductors, said inner wall and the other of said outer walls being spaced more closely than said one outer wall and saidv inner wall, and said inner wall forming with said outer walls a plurality of serially connected radial transmission line sections resonant at the frequency of the wave transmitted over said concentric line.

GEORGE E. FEIKER, Jn.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,171,219 Malter Aug. 29, 1939 2,332,952 Tischer Oct. 26, 1943 2,408,927 Gurewitsch Oct. 8, 1946 2,415,962 Okress Feb. 18, 1947 2,419,613 Webber Apr. 29, 1947 2,434,610 Feiker Jan. 13, 1948 2,438,912 -Hansen Apr. 6, 1948 2,456,803 Wheeler Dec. 21, 1948 2,456,896 Slack Dec. 21, 1948 

